It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 10 .mu.m average by area size as measured by a Horiba CAPA-500 centrifugal automatic particle analyzer After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said nonpolar liquid and the image may subsequently be transferred to a carrier sheet. Developed images of two or more colors can be prepared by creating successive, single color light images, successively recording these images on the photoconductive surface, developing the photoconductive surface with liquid electrostatic developer containing colored toner particles complementary in color to the color light image, and transferring to a carrier sheet or receptor support each developed image in either superimposed registration or in non-overlapping relation to its preceding transferred image. This type of process using liquid electrostatic developer is time-consuming because of the many steps involved and registration or location of the transferred images can be a problem.
In many instances only two-color images are desired. While these can be of any possible color combination, frequently the colored images desired by business are black and red. Dry toners or developers consisting of two colored pigments such as black and red have been mixed together and applied either successively or simultaneously to a conductive surface and subsequently transferred to a receptor support. Dry toner combinations rely mainly on their placement in the triboelectric series for their particular charge. Such dry toner compositions have certain disadvantages relative to liquid electrostatic developers, e.g., low resolution due to larger particle size, less suitable for high speed copying due to slower development times, limited color gamut and limited colors due to difficult pigment dependent charging, and difficult removal of background toner. However, the use of known liquid electrostatic developers presents other disadvantages, e.g., inability to mix color toners of any color without cross color contamination, inability to mix different liquid toners with different charge polarity, and inability to control charging independent of pigment.
It is desired to provide a bipolar liquid electrostatic developer which can overcome the aforementioned disadvantages.